This invention relates to an apparatus for advancing individual letters or similar flat items from a stack formed by such articles. The apparatus is of the type that has a revolving withdrawing mechanism which is in frictional contact with the foremost item in the stack and which individually advances the items past a retaining mechanism to a predetermined standby position. Upon a call signal, the items are transported from the standby position to a feed mechanism, comprising, for example, a pair of continuously driven rollers which, in turn, advance the items to a further conveyor path.
An apparatus of the above-outlined type is described, for example, in U.S. Pat. No. 3,339,917. If the withdrawal of a flat item from the stack is effected without the use of air suction and there is used only a withdrawing mechanism that frictionally engages the leading item of the stack (as disclosed in the above-mentioned patent), then, particularly in the further processing of letters which usually have different weights and frictional properties, one has to take into account that the time necessary for the removal of the item from the stack and its acceleration to the circumferential speed of the withdrawing mechanism are not constant. In known devices these time differences are eliminated by advancing each item only up to a predetermined standby position and then -- in their already separated condition -- they are subsequently "called" from the standby position. Thus, the items are not called directly from the stack.
In the known apparatus, the standby position is constituted by an abutment controlled by the call signal. The distance of the abutment from the withdrawing mechanism is preferably smaller than the length of the shortest item. The advancing force of the withdrawing mechanism is a continuously rotating roller provided with a friction surface, may therefore be only so large that even a thin item, when it is retained by its leading edge at the abutment, is not advanced in the zone of the withdrawing mechanism and thus does not undergo buckling or crumpling. This measure, however, may give rise to difficulties when heavier items have to be withdrawn from the stack. Further, the structural design of the controlled abutment also involves problems involving wear and operational noise.